1. Field of The Present Invention
The present invention relates generally to power transfer apparatus for motor vehicles and, more particularly, to an automatic transmission having an overdrive ratio.
2. Description Of The Prior Art
A known production automatic overdrive transmission includes four forward speed ratios including a direct drive ratio and an overdrive ratio in which the input member turns at a slower speed than the output member due to the selected torque path. In this production transmission, the shift between the direct drive ratio and the overdrive ratio requires synchronous engagement of one clutch member while another clutch member is disengaged. Such synchronizing control is provided by a hydraulic control circuit. U.S. Pat. No. 3,314,307 describes such a transmission.
As demonstrated in the following chart, a plurality of shifter positions can be selected, for example, reverse (R), overdrive range (OD), overdrive lock-out range (D), and low gear (U), as well as park and neutral positions (not shown). The engagements of the clutches defining the torque paths are illustrated in the chart below. In the overdrive range, disengagement of the forward clutch must occur in synchronization with the application of the overdrive band so as to reduce abrupt (tie-up) or unduly extended (engine runaway) load changes on the engine and transmission components.
__________________________________________________________________________ CHART A 30 32 34 44 24 40 42 24 Shift Int. Int. O.D. Rev. Fwd. Low Low/Rev. Dir. Posit'n Gear Cl. O.W.C. Band Cl. Cl. O.W.C. Band Cl. __________________________________________________________________________ OD. 1 x x 2 x x x OR 3 x OR x OR x 4 x x OR x D. 1 x x 2 x x x OR 3 x OR x OR x 1 1 x x x R x x __________________________________________________________________________
Such synchronous shifting provides several disadvantages. In particular, gradual engagement of the overdrive band increases wear on the band and reduces its durability. Moreover, the transmission referred to above includes an open converter torque path through the forward clutch as well as a direct lock-up torque path through the direct clutch in the direct drive (third gear) ratio. As a result, additional wear is imposed upon the direct clutch, the overdrive band and the forward clutch and, during a 3-4 shift, adversely affects the clutch's durability. Furthermore, tie-up and runaway can only be reduced, but not avoided, in synchronous shifting. Accordingly, the synchronous engagement and disengagement of the separate clutches, which generally takes about one-half second to complete, is quite noticeable to the vehicle operator.
Although series connections of a friction clutch and a one-way clutch are employed in the production version to provide 1-2 and 2-3 shifts nonsynchronously, a proposal to use an additional one-way clutch creates a severe packaging problem. In particular, one-way clutches occupy a substantial amount of space. In addition, such clutches also interrupt reversal of the torque path which permits the engine to act as a brake when deceleration is desirable. Consequently, torque path reversal requires the addition of an additional clutch and its associated controls. Accordingly, the design criteria of drive-line function and packaging deter the skilled artisan from incorporating a serial friction clutch and one-way clutch arrangement for nonsynchronous 3-4 shifts.
U.S. Pat. No. 4,226,123 to Croswhite discloses a transmission in which shifts between the forward speeds, including an overdrive ratio, use one-way clutches for nonsynchronous shifting. However, the forward drive clutch coupled in series with the one-way clutch is positioned axially adjacent to a hill brake clutch assembly. In addition, each of the clutches is operated by a separate piston, which further adds to the complexity, weight and size of the transmission. Moreover, the forward clutch provides an open converter torque path to the forward sun gear so that the sun gear turns at substantially the same speed as the planet carrier coupled to the direct drive shaft by the direct clutch. However, due to torque converter slip, reduced fuel economy accompanies the torque availability in the direct (third gear) drive ratio.
In view of the limited areas in which a transmission can be mounted in the vehicle without obstructing passenger comfort, packaging considerations are often important factors in determining whether new transmission arrangements can be put into production. In particular, re-sizing of a transmission housing can deter the introduction of production transmissions due to extended lead times in the design and preparation for production of the transmissions. The incorporation of additional components to be driven or carried within the transmission also increases fuel consumption and further deters the skilled artisan from redesigning production transmissions to include them.